Where a user interface device, such as a mobile phone, has been configured to receive and process requests for authorisation of use of a credential, such as a credit card, it has been realised that it would be desirable to alert the person who controls the mobile phone to the pending transaction by means of a signal which is emitted by the phone which is indicative of the monetary value, say, of the transaction. The control of the provision of authorisation for use of the credential is thus facilitated.
The term credential is used herein to denote any means which provides prima facie authority for entitlement to do something, for example a credit or debit card paying for goods or a pass card or PIN for entering a particular region of a building.